Biology would have been a good one but if that's totally out of the question, then I would suggest going for your strongest subject to give you the best chance of a set of decent grades. In addition to this, I would be looking to undertake relevant experience so that when you apply to uni in a couple of years, you can write a good personal statement. For example, voluntary work in your local community where you have some involvement in projects which support vulnerable groups.
An afterthought - at degree level, you will use SPSS (statistical software) so Maths might be a good choice?